JP5192610B2 - MEMS element and method for manufacturing MEMS element - Google Patents

Abstract

In a MEMS device having a substrate 1, a sealing membrane 7, and a movable portion 3 of beam and an electrode 5 which have a region wherein they overlap with a gap in perpendicular to a substrate 1 surface, a first cavity 9 is on the side of the movable portion 3 in the direction perpendicular to the surface of the substrate, and a second cavity is the other cavity, and an inner surface a of a side wall A in contact with the electrode 5, of the first cavity 9, is positioned more inside than an inner surface b of a side wall B in contact with the electrode 5, of the second cavity 10, in the direction parallel to the substrate surface, such that the movable portion 3 does not collide with the electrode 5 when mechanical stress is applied from outside to the sealing membrane 7.

Description

  The present invention relates to a MEMS (Micro-Electro Mechanical Systems) element, and more particularly, to a MEMS resonator that has a minute gap structure and realizes highly reliable sealing.

  An example of a conventional MEMS resonator will be described with reference to FIGS. 7 and 8 are a perspective view and a cross-sectional view showing a structure of a MEMS oscillator manufactured using an SOI (Silicon on insulator) substrate disclosed in Patent Document 1. FIG. This MEMS resonator is used in, for example, a filter as described in Patent Document 1. Here, the SOI substrate is a substrate manufactured by forming a device formation layer made of a single crystal silicon layer on a silicon substrate through a BOX layer (buried silicon oxide film) made of a silicon oxide film. .

  In the manufacture of the MEMS resonator shown in FIG. 7, anisotropic etching is first performed on an SOI substrate to form a beam having a triangular cross section (triangular cross section beam), and a silicon oxide film for forming a gap is formed. The electrode 202 is then formed. Thereafter, the silicon oxide for the gap and the BOX layer 206 are removed, leaving a portion to be a support portion. Thereby, the triangular cross-section beam serving as the vibrator 201 is opened so as to be movable, and an air projecting structure in which electrodes having a space (cavity) and a narrow gap are arranged on the side surface of the triangular cross-section beam having a projecting structure. Department is completed.

  As shown in FIG. 8, a space (cavity) 207 is formed under the vibrator 201. By this manufacturing method, a MEMS resonator having a vibrator made of single crystal silicon using an SOI substrate and electrode terminals that enable electrostatic excitation and electrostatic detection is realized. In this manufacturing method, since the gap forming film and the BOX layer 206, which is the lower layer of the vibrator 201, are both silicon oxide films, gap formation and structure opening can be performed simultaneously in the final release (structure opening) step. Reduction of the manufacturing process is realized. Patent Document 1 discloses a method of covering the resonator with a glass cap.

  A vibration type pressure sensor having a conventional sealing structure will be described with reference to FIG. FIG. 9 is a cross-sectional view of a pressure sensor manufactured using the MEMS technology described in Patent Document 2. The vibrator 103 is a beam made of single crystal silicon. The vacuum chamber 105 is formed by a sacrificial layer etching technique using a difference in etching rate caused by the impurity concentration of epitaxially grown silicon.

  The shell 104 is also formed by a thin film forming technique. An electrostatic capacitance is formed between the shell 104 and the beam (vibrator) 103. Both ends of the beam are anchored to the measurement diaphragm and can vibrate near the resonance frequency. The element shown in FIG. 9 exhibits a function as a pressure sensor by capturing a change in the stress of the beam due to the pressure applied to the measurement diaphragm as a change in resonance frequency.

  The Q value representing the sharpness of resonance of the beam is deteriorated by the viscosity of the air around the beam. Therefore, a high Q value can be maintained by keeping the vacuum chamber in a vacuum. The higher the Q value, the more sensitively senses the resonance frequency change due to pressure.

  The pressure sensor described with reference to FIG. 9 can be manufactured by a method of sealing a resonating beam to a vacuum only by a thin film process. Therefore, in the manufacture of this pressure sensor, the vacuum sealing process in the element packaging process is not required, and it is possible to provide a small pressure sensor at low cost.

US Pat. No. 7,358,648 JP 2005-37309 A

  The sealing technique based on the thin film process described in Patent Document 2 can be applied not only to a pressure sensor but also to a resonator, a filter, an oscillator, a switch element, a gyroscope, a mass detection element, and the like using a MEMS technique. . The purpose of sealing is not only to keep the vibrator movable in a vacuum. In an element that does not require a vacuum, the technique described in Patent Document 2 is used for the purpose of isolating the humidity and dust from the outside of the seal, or protecting the inside of the seal from the resin filling pressure at the time of packaging by a resin transfer mold. Can be applied.

  However, the sealing technique of Patent Document 2 cannot be applied to a MEMS resonator as described in Patent Document 1 as it is. Therefore, sealing by resin transfer molding, which is generally performed in the field of semiconductor elements, is considered as a sealing method. However, the sealing of the MEMS resonator described in Patent Document 1 by the resin transfer mold has the following problems.

  Specifically, at the time of resin transfer molding, there is a problem in that the structure to be sealed (particularly the electrode) itself is greatly distorted due to the pressure applied from the outside, and the electrode and the beam (movable part) collide. In the MEMS resonator, it is necessary to narrow the gap between the electrode and the beam (vibrator) in order to reduce the element impedance. When the MEMS resonator described in Patent Document 1 is used as a timing device of an electronic apparatus, the gap is about 100 to 200 nm, and such a narrow gap can be maintained under pressure during resin transfer molding. become unable.

  In order to avoid this problem, as shown in FIG. 10, there is a method in which a sealing structure is formed separately from the electrodes, and cavities are formed above and below the electrodes 305 and the movable portion (vibrator) 303. In FIG. 10, reference numeral 301 denotes a substrate, 302 and 306 denote sacrificial layers, and 307 denotes a sealing thin film. The cavities are formed by partially removing the sacrificial layers 302 and 306, and the remaining portions of the sacrificial layers 302 and 306 constitute side walls that define the cavities. According to this method, a sealing structure can be obtained with the sealing thin film 307. However, the inventors have newly found that even when the structure shown in FIG. 10 is adopted, the following problems occur when this element is further sealed with a resin transfer mold.

  Specifically, in the MEMS resonator having the configuration shown in FIG. 10, when the cavity B on the electrode side is inside the side wall A of the cavity on the movable part side, the pressure applied during the resin transfer molding is transmitted to the sealing thin film. The force is further transmitted to the electrode through the cavity side wall. The direction of the force applied to the electrode is parallel to the thickness direction of the element. Therefore, if the side wall of the cavity on the electrode side is inside the side wall of the cavity on the movable part side, that is, if there is a space below the side wall on the electrode side, the electrode bends due to the force applied downward to the electrode. As a result, as shown in FIG. 10, the electrode may move downward and collide with the movable part.

  FIG. 11 is a graph showing the result of calculating the strain amount in the thickness direction of the sealing thin film when an external pressure is applied. Each line shows the difference in the film thickness of the sealing thin film, and the larger the film thickness, the smaller the strain amount. In the case of resin transfer molding, a pressure of about 1E + 07 Pa (100 atm) to 1.5E + 07 Pa (150 atm) is applied to the sealing thin film. Therefore, even if the thickness of the sealing thin film is as thick as 4 μm, it is distorted by about 300 nm under 150 atm.

  The external pressure that causes such a large strain is such that the electrode and the movable part collide with each other. That is, the MEMS resonator shown in FIG. 10 has a structure in which the force applied to the sealing thin film is transmitted to the electrode as it is and the electrode is bent. Therefore, for example, if the gap between the electrode 305 and the movable part 303 is about 100 to 300 nm, the gap does not hold in the hatched part in FIG. 11 and the electrode and the movable part collide. In order to avoid collision, it is necessary to use a sealing thin film having a thickness of at least 6 to 7 μm. However, as long as the current semiconductor thin film formation technology is applied, a film having such a thickness can be formed only by laminating a large number of thin films, which includes another problem that the throughput is deteriorated. Yes.

In order to solve the above problems, the present invention provides a MEMS element having a structure that prevents stress from being applied to an electrode in a direction in which the electrode approaches a movable part when an external pressure is applied during resin transfer molding or the like. To do. That is, the present invention
Having a substrate and a sealing thin film,
A movable part that performs mechanical vibration and an electrode positioned in proximity to the movable part are provided between the substrate and the sealing thin film, and the movable part and the electrode have a gap in a direction perpendicular to the substrate surface. Having regions that overlap each other,
A first cavity and a second cavity separated by the electrode are formed between the substrate and the sealing thin film,
The first cavity is located on the movable portion side in a direction perpendicular to the substrate surface when viewed from the electrode in the region where the movable portion and the electrode overlap.
The second cavity is located on a side opposite to the movable part in a direction perpendicular to the substrate surface when viewed from the electrode in a region where the movable part and the electrode overlap.
The inner surface of the side wall A in contact with the electrode of the first cavity is located on the inner side in the direction parallel to the substrate surface than the inner surface of the side wall B in contact with the electrode of the second cavity.
A MEMS device is provided.

The MEMS element of the present invention has two cavities (a first cavity and a second cavity) that are separated from each other by an electrode, and an inner surface of a side wall A that is in contact with the electrode of the first cavity is The first cavity is located on the inner side in the direction parallel to the substrate surface than the inner surface of the side wall B in contact with the electrode of the second cavity, and the first cavity is from the electrode in the region where the movable part and the electrode overlap. look, positioned on the side of the movable portion in a direction perpendicular to the substrate surface, a second cavity, as viewed from the electrode in the region where the electrode and the movable portion overlaps, a direction perpendicular to the substrate surface It is located in the opposite side to the said movable part in. With this feature, even when mechanical pressure is applied to the sealing thin film, it is possible to prevent the electrode from being stressed in a direction approaching the movable part. As a result, the collision between the electrode and the movable part is avoided, and the gap between the electrode and the movable part is maintained. Moreover, in the MEMS element of the present invention, since it is not necessary to reduce the amount of strain when the sealing thin film is thickened and an external pressure is applied, the sealing thin film has a small film thickness of 2 μm or less, for example. can do. The use of the sealing thin film having such a small thickness contributes to the reduction of the manufacturing time of the MEMS element.

  The present invention also provides a method for manufacturing the MEMS device of the present invention having the above structure. The manufacturing method of the present invention includes forming the first cavity by removing the first sacrificial layer, and forming the second cavity by removing the second sacrificial layer. The removal of the two sacrificial layers may be performed within the same etching process, with the second sacrificial layer removed first and then the first sacrificial layer removed, or within the same etching process. The first sacrificial layer may be removed first and then the second sacrificial layer may be removed. When the first sacrificial layer is first removed, the etching rate of the second sacrificial layer is higher than the etching rate of the first sacrificial layer by using the materials of the first sacrificial layer and the second sacrificial layer. To ensure that the inner surface of the sidewall in contact with the electrode of the second cavity is located more outward in the direction parallel to the substrate surface than the inner surface of the sidewall in contact with the electrode of the first cavity. There is a need.

  The MEMS element of the present invention is preferably provided in a form in which the outside of the sealing thin film is molded with a resin. A rigid sealing structure is realized by the resin mold. As described above, the MEMS element of the present invention has a structure that avoids collision between the electrode and the movable part due to pressure applied during resin molding, and is therefore suitable for sealing with resin molding.

FIG. 1 is a cross-sectional view showing an example of the sealing structure of the MEMS element according to the first embodiment of the present invention. 2A to 2C are cross-sectional views showing an example of a structure in which the MEMS element according to the first embodiment of the present invention is molded and sealed with a resin. FIGS. 3A to 3E are cross-sectional views showing a process flow of the method for manufacturing a MEMS device according to the first embodiment of the present invention. FIG. 4 is a cross-sectional view showing an example of the sealing structure of the MEMS element according to the second embodiment of the present invention. FIG. 5 is a cross-sectional view showing an example of the sealing structure of the MEMS element according to the third embodiment of the present invention. FIG. 6 is a sectional view showing another example of the MEMS element sealing structure according to the third embodiment of the present invention, in which a through hole is formed in a substrate. FIG. 7 is a perspective view of a conventional triangular beam torsion resonator. FIG. 8 is a cross-sectional view of a conventional triangular beam torsion resonator. FIG. 9 is a cross-sectional view showing a conventional MEMS element sealing structure. FIG. 10 is a cross-sectional view showing a thin film sealing structure of a conventional MEMS element. FIG. 11 is a graph showing the relationship between the external pressure applied to the sealing thin film and the amount of strain in the thickness direction of the sealing thin film. FIG. 12 is a cross-sectional view illustrating a part of the MEMS element illustrated in FIG. 1 and illustrating O, A1, B1, C1, D1, E1, and F1. FIG. 13 is a graph showing E1 / C1 and F1 / D1 when B1 / A1 is changed in FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 is a cross-sectional view showing a structural example of a MEMS element according to the first embodiment of the present invention. In the illustrated MEMS device, two sacrificial layers 2 and 4 are formed on a substrate 1, and a movable part which is a beam structure on the lower first sacrificial layer 2 in FIG. 1. 3 is formed, and an electrode 5 is formed on the first sacrificial layer 4 on the upper side. The movable part 3 and the electrode 5 have a region overlapping each other with a gap in the direction perpendicular to the surface of the substrate 1 (that is, in the illustrated form, the main surface of the movable part 3 and the main surface of the electrode 5 are , Facing each other so as to be parallel to the substrate surface), the vibrator is configured. The movable portion 3 has a structure in which a beam extending in a direction perpendicular to the paper surface can vibrate and its end portion is fixed. The movable part 3 and the electrode 5 form a capacitance by a minute gap, and the movable part 3 is excited in a mode such as bending, spreading, or twisting by an electrostatic force generated by applying a voltage to the electrode 5. Here, the “direction perpendicular to the substrate surface” can also be said to be the thickness direction of the substrate (and thus the element). Therefore, it can be said that the “direction perpendicular to the substrate surface” is a direction in which films are stacked in the semiconductor thin film formation technology.

  In the illustrated form, the cavity located on the movable part 3 side in the thickness direction of the substrate 1 is the first cavity when viewed from the electrode 5 in the region where the electrode 5 and the movable part 3 overlap. Therefore, in the illustrated form, a cavity located in the lower direction when viewed from the electrode 5, that is, a space indicated by reference numeral 9 is the first cavity. Here, the first cavity 9 is formed by being surrounded by the substrate 1, the first sacrificial layers 2, 4 and the electrode 5. The first sacrificial layers 2 and 4 shown in FIG. 1 are portions that have not been removed by etching, and these portions define side surfaces (surfaces in a direction perpendicular to the substrate surface) of the first cavities 9. The side wall is configured. The side wall shown in FIG. 1 is a side wall in contact with the electrode 5 located on both sides of the beam constituting the movable part 3 in the drawing, that is, on both sides of the beam extending in a direction parallel to the direction in which the beam extends (FIG. 1). In A). As shown in the figure, when the sacrificial layer is composed of two or more layers and the inner surface of each layer is in a different position, the inner surface of the sacrificial layer on the side close to the electrode, that is, the side in contact with the electrode, The position (position indicated by a in FIG. 1) is determined in relation to the inner surface of the side wall B of the second cavity.

  A second sacrificial layer 6 is formed on the electrode 5, and a sealing thin film 7 is further formed thereon. As described above, the first cavity is a cavity located on the movable part 3 side, that is, the lower side when viewed from the electrode 5, so the second cavity is the movable part 3 when viewed from the electrode 5. It is a cavity located on the opposite side, that is, the upper side. Therefore, in the illustrated form, the space indicated by reference numeral 10 is the second cavity. Here, the second cavity 10 is formed by being surrounded by the electrode 5, the second sacrificial layer 6, and the sealing thin film 7. The second sacrificial layer 6 shown in FIG. 1 is a portion that has not been removed by etching, and this portion constitutes a side wall that defines the side surface of the second cavity 10. The side walls shown in FIG. 1 are side walls (indicated by B in FIG. 1) that are in contact with the electrode 5 and are located on both sides when viewed from the direction in which the beams constituting the movable part 3 extend. When the second sacrificial layer is composed of two or more layers and the inner surface of each layer is in a different position, the position of the inner surface of the layer on the side close to the electrode, that is, the side in contact with the electrode (see FIG. The position indicated by b in FIG. 1 is considered in determining the position of the inner surface of the side wall A of the first cavity.

  A through-hole 8 is formed in the sealing thin film 7. The through-hole 8 is formed at a desired position as an etching hole in order to form a cavity by removing the sacrificial layer. The electrode 5 is also formed with a through hole 12 for removing the first sacrificial layers 2 and 4. The first sacrificial layers 2 and 4 and the second sacrificial layer 6 may be formed of the same or the same material. In the etching, an etching gas or the like is introduced from the through-hole 8 to remove the second sacrificial layer 6, and the etching gas or the like is further passed through the through-hole 12 to pass through the first sacrificial layer 4 and the first sacrificial layer. You may implement by the method of removing 2. FIG. That is, the second sacrificial layer 6, the first sacrificial layer 4, and the first sacrificial layer 2 may be removed in this order. In that case, for example, when the first sacrificial layer 4 is etched, the second sacrificial layer 6 is also simultaneously eroded, so that the second cavity 10 is more parallel to the substrate surface than the first cavity 9. Has a large dimension in the direction (horizontal direction). That is, the inner surface a of the side wall A that defines the first cavity 9 is located on the inner side in the direction parallel to the substrate surface (horizontal direction) than the inner surface b of the side wall B that defines the second cavity 10. .

  The shape and number of the through holes 8 and the through holes 12 are not particularly limited as long as they function as desired as etching holes. For example, a plurality of through-holes 8 having a substantially circular shape when viewed from above may be provided in the sealing thin film 7. Further, as the through-hole 12, one having a substantially circular shape or a substantially rectangular shape when viewed from above may be provided, or a plurality thereof may be provided along a direction parallel to the direction in which the beam of the movable portion 3 extends. Alternatively, as the through-hole 12, a slit-like opening when viewed from above may be provided over the entire electrode 5 in a direction parallel to the direction in which the beam of the movable portion 3 extends. In that case, the electrode 5 has a configuration in which the electrode 5 is separated into two parts with the slit-shaped through hole 12 as a boundary, and the two parts are both the second sacrificial layer 6 and the first sacrificial layers 2, 4. It becomes a cantilever fixed between.

  FIG. 2A shows a form in which the surface of the sealing thin film 7 of the MEMS element shown in FIG. The pressure applied to the sealing thin film 7 when carrying out the transfer molding of the resin 11 is applied to the side wall B of the second cavity as shown in FIG. Push in. However, since the first sacrificial layer 4, the first sacrificial layer 2, and the substrate 1 exist as a laminated structure and are fixed under the electrode 5, the pressed-in electrode 5 is only on the surface side. Distorted. As a result, in the vicinity of the contact point between the electrode 5 and the side wall B, stress is generated in the direction toward the contact point, and in the hollow part (unfixed part) of the electrode 5, as shown in FIG. The surface side is pulled in the contact direction. Furthermore, since the lower side of the contact between the electrode 5 and the side wall B is fixed to the side wall A (first sacrificial layer 4) of the first cavity, the electrode 5 has a hollow portion facing upward (away from the movable part 3). Direction). Due to the generation of such stress, the collision between the electrode 5 and the movable portion 3 is avoided, and the gap is maintained.

  How much the inner surface a of the side wall A of the first cavity is positioned inside the inner surface b of the second cavity side wall B depends on the size and shape of the element, the position and shape of the electrode and the movable part, and It is preferable to appropriately determine the size and the mechanical stress generated in the electrode when pressure is applied to the sealing thin film. Hereinafter, how much the inner surface a of the side wall A of the first cavity is positioned inside the inner surface b of the second cavity side wall B will be described using simulation results.

  FIG. 13 shows the result of simulating the maximum displacement in the thickness direction of the electrode 5 and the sealing thin film 7 with respect to the position of the side wall B of the second cavity when 100 atm is applied to the sealing thin film 7 as the mold pressure. . FIG. 12 shows A1 and B1, which are variables when simulating the maximum displacement in the thickness direction of the electrode 5 and the sealing thin film 7, the distance C1 between the electrode 5 and the movable part 3, and the electrode 5 and the sealing thin film 7. It is sectional drawing explaining distance D1 between these, and is equivalent to a part of sectional drawing shown in FIG.

  In FIG. 12, the position indicated by O is the reference position of the electrode 5. Here, the reference position of the electrode refers to a region facing the movable part when pressure (mold pressure) is applied to the sealing thin film during transfer molding, thereby applying force in the thickness direction to the electrode and displacing the electrode. The point where the displacement in the thickness direction of the electrode becomes maximum. Therefore, as in the MEMS element shown in FIG. 1, the opening 12 is a slit-like opening, and the longer tip position of the separated cantilever electrodes coincides with the side edge of the movable part. In other words, the reference position O is the longer tip (that is, the boundary with the opening 12). When the opening 12 is circular, the electrode 5 is not divided into two with respect to the opening, and the side wall B of the second cavity is in a symmetrical position with respect to the movable part 3, The position facing the center of the movable part 3 is determined as the reference position O. That is, in the cross-sectional view, assuming that the electrode is separated into two cantilevers with the opening as a boundary, the tip of the longer cantilever is set as the reference position O.

  A1 is a distance from the reference position O to the side wall A of the first cavity in a direction parallel to the substrate surface and perpendicular to the direction in which the beam of the movable part 3 extends, and B1 is parallel to the substrate surface. The distance from the reference position O to the side wall B of the second cavity in a direction perpendicular to the direction in which the beam of the movable part 3 extends. When the side wall A of the first cavity is not parallel to the direction in which the beam of the movable part 3 extends (for example, when the side wall A is curved when the side wall A is viewed from above), the side wall The reference position O is determined according to the shape of A and the position of the opening 12, and accordingly, A1 (the distance in the predetermined direction between the position and the side wall A at the position where the displacement of the electrode in the thickness direction is the largest). ) Is determined. The same applies to B1. C1 is the shortest distance between the electrode 5 and the movable part 3 in the direction perpendicular to the substrate surface, and D1 is the shortest distance between the electrode 5 and the sealing thin film 7 in the direction perpendicular to the substrate surface. It is.

  In the simulation, when B1 / A1 is changed, the maximum displacement in the direction perpendicular to the substrate surface of the electrode 5 (ie, the thickness direction) is E1, and the direction perpendicular to the substrate surface of the sealing thin film 7 (ie, the thickness direction). ) Was defined as F1, and E1 / C1 and F1 / D1 were obtained by calculation. In FIG. 12, the horizontal axis is B1 / A1, the vertical axis (left axis) is E1 / C1, and the vertical axis (right axis) is F1 / D1. The solid line (left axis) represents the maximum displacement in the thickness direction of the electrode 5 when the positional relationship between the side walls of the first and second cavities is changed, and the broken line (right axis) represents the first and second cavities. It represents the maximum displacement in the thickness direction of the sealing thin film 7 when the positional relationship of the side walls is changed. Here, when E1 / C1 (left axis) is in the positive region, it means that the electrode 5 moves away from the movable portion 3, and when E1 / C1 is in the negative region, the electrode 5 approaches the movable portion 3. Means that. Therefore, E1 / C1 = −1.0 represents that the electrode 5 and the movable part 3 are in contact with each other. Similarly, when F1 / D1 (right axis) is in the positive region, the sealing thin film 7 moves away from the electrode 5, and when F1 / D1 is in the negative region, the sealing thin film 7 approaches the electrode 5. Means that. Therefore, F1 / D1 = −1.0 represents that the sealing thin film 7 and the electrode 5 are in contact with each other.

  When B1 / A1 (horizontal axis) is 1.0, that is, when A1 = B1, the position of the side wall A of the first cavity and the position of the side wall B of the second cavity coincide with each other. Therefore, B1 / A1> 1.0 indicates that the side wall A of the first cavity is inside the side wall B of the second cavity, and B1 / A1 <1.0 indicates that the side wall A of the first cavity The side wall A is located outside the side wall B of the second cavity.

As a result of the simulation of FIG. 12, it was confirmed that E1 / C1 (solid line) is a positive value of 0 or more in a region where B1 / A1 is larger than 1.0. That is, by setting the side wall A of the first cavity to be inside the side wall B of the second cavity, the gap between the electrode 5 and the movable part 3 can be maintained, or the electrode 5 can be moved to the movable part. Can be kept away from 3. However, in consideration of the mask position accuracy of the through hole (etching hole) and the variation in etching, the position of the inner surface b of the side wall B may be 0.1 μm or more outside the position of the inner surface a of the side wall A. preferable.
E1 / C1 exhibits a peak when B1 / A1 is near 1.1, and approaches 0 when B1 / A1 ≧ 1.5. This is because, as shown in the FIG. 2 (b) (c), the electrode 5 and the distortion surface of the electrode 5 in the vicinity of the contact of the side wall B, Oite the electrode 5, the reference position O the movable part 3 of the electrode 5 This shows that the effect of applying stress in the direction away from the distance increases between B1 / A1 = 1.0 and B1 / A1 = 1.5.

FIG. 12 shows that when B1 / A1 is 1.5 or more, the stress applied to the electrode 5 becomes small, and the reference position O of the electrode 5 is hardly displaced in the thickness direction. That is, when B1 / A1 is 1.5 or more, even if the mold pressure is applied to the sealing thin film 7, the distance between the electrode 5 and the movable part 3 changes almost from that when the mold pressure is not applied to the sealing thin film 7. I can say no. This means that when B1 / A1 ≧ 1.5, the gap between the electrode 5 and the sealing thin film 7 is kept substantially constant regardless of whether or not the mold pressure is applied. Therefore, it is preferable that B1 / A1 is 1.5 or more. This is because the difference between the value of the gap between the electrode and the sealing thin film while the mold pressure is applied and the design value of the gap can be reduced or eliminated.

  F1 / D1 indicated by a broken line is in a negative region smaller than 0 regardless of the value of B1 / A1. This indicates that the sealing thin film 7 is displaced by application of mold pressure regardless of how the side wall A of the first cavity is arranged with respect to the side wall B of the second cavity. Also, according to FIG. 12, it can be seen that the amount of distortion of the sealing thin film 7 in the direction approaching the electrode 5 increases as B1 / A1 increases. Further, FIG. 12 shows that when B1 / A1 takes a value near 3.2, F1 / D1 becomes −1.0, that is, the sealing thin film 7 and the electrode 5 are in contact with each other. When the sealing thin film 7 and the electrode 5 are in contact with each other, the resonator operation cannot be ensured, so that B1 / A1 is preferably set to 3.2 or less.

  Further, regardless of the size of the first cavity, if the size of the second cavity 10 is large, the sealing thin film 7 bends and contacts the electrode 5 and / or the movable part 3 during the resin transfer molding. There is. Therefore, when the pressure of about 100 to 150 atm is applied in consideration of the gap between the electrode 5 or the movable part 3 and the sealing thin film 7, the Young's modulus and the film thickness of the sealing thin film 7, the sealing thin film 7 It is preferable to determine the dimension of the second cavity and thus the position of the inner surface of the side wall B to such an extent that it does not contact the electrode 5 and / or the movable part 3.

  For example, when the sealing thin film 7 is made of SiGe having a thickness of 10 μm or less and the gap between the electrode 5 and the sealing thin film 7 is 1 μm, the width of the second cavity 10 (the surface of the substrate 1 is When the x-y coordinate plane and the direction parallel to the direction in which the beam of the movable part 3 extends is the y direction, the dimension in the x direction is about 200 μm, and the mold pressure is 100 to 150 atm. The stop film 7 is in contact with the electrode 5 and / or the movable part 3. Therefore, in the MEMS element having such a configuration, it is preferable to determine the position of the inner surface of the sidewall B so that the width of the second cavity is less than 200 μm.

  In the MEMS element shown in FIG. 1, the side wall A that defines the first cavity 9 is formed of two layers having steps. As will be described later, this occurs, for example, by manufacturing using an SOI substrate and without providing an etching stop. In the modification of the MEMS element shown in FIG. 1, the inner surfaces of the two first sacrificial layers may be in the same position, or the inner surface of the lower first sacrificial layer 2 may be the upper sacrificial layer 4. It may be located outside the inner surface in a direction parallel to the substrate surface.

  In the MEMS element of the present invention, as described above, of the side walls defining the first cavity, the inner surface of the side wall in contact with the electrode is closer to the substrate surface than the inner surface of the side wall in contact with the electrode of the second cavity. It is necessary to be located inside in the parallel direction. In the MEMS element shown in FIG. 1, the side walls A and B in contact with the electrodes of the first and second cavities are located on both sides of the beam of the movable part 3. For example, the cavity shown in FIG. 1 is viewed from above. If it is generally rectangular, it is located along a direction parallel to the direction in which the beam extends. Therefore, in the other side wall, for example, in the MEMS element shown in FIG. 1, when the cavity is substantially rectangular when viewed from above, the beam of the movable part 3 is not shown in FIG. The side wall of the first cavity, which is located along a direction substantially parallel to the vertical direction (the left-right direction in the figure), is a second position located along the direction perpendicular to the direction in which the beam of the movable part 3 extends. It may be on the outside of the inside surface of the sidewall of the cavity, or on the inside. However, when the MEMS device of the present invention is manufactured using the process described later, generally, the inner surface of the entire side wall of the first cavity is located inside the inner surface of the entire side wall of the second cavity. It will be.

  FIG. 3 is a process flow diagram showing an example of a method for manufacturing the MEMS element of FIG. First, as shown in FIG. 3A, from a silicon substrate 31, a BOX layer (buried silicon oxide film) 32 (corresponding to 2 in FIG. 1), and a single crystal silicon layer 33 (corresponding to 3 in FIG. 1). An SOI substrate is prepared. In this SOI substrate, a photolithography process and an etching process are performed, and the single crystal silicon layer 33 is patterned. Next, as shown in FIG. 3B, a silicon oxide film 34 (corresponding to 4 in FIG. 1) is formed. Next, as shown in FIG. 3C, as the conductive layer 35 (corresponding to 5 in FIG. 1), for example, a metal material such as Pt or Al, or porous silicon is formed, and a desired pattern is formed. Patterning is performed by a photolithography process and an etching process so as to obtain the conductive layer 35 having the same.

Further, as shown in FIG. 3D, on the conductive layer 35, a silicon oxide film 36 (corresponding to 6 in FIG. 1) and a film 37 of a material for forming a sealing thin film (corresponding to 7 in FIG. 1). ) Are sequentially formed. Here, the material for forming the film 37 is selected from materials having resistance in the sacrificial layer removal step, such as porous silicon, Pt, Al, and Al ₂ O ₃ . Finally, as shown in FIG. 3E, a through-hole 38 (corresponding to 8 in FIG. 1) is opened in the film 37 by a photolithography process and an etching process, and an etching gas or an etchant is introduced from the through-hole, Desired regions of the silicon oxide film 36, the silicon oxide film 34, and the BOX layer 32 are sequentially removed. The structure of FIG. 1 is implement | achieved by implementing the above process. According to this manufacturing method, even when the films 36 and 34 and the layer 32 are made of the same or similar material, the first cavity and the second cavity can be formed by one etching process without requiring special control. The inner surface of the entire side wall of the first cavity may be formed so as to be located inside the inner surface of the entire side wall of the second cavity in a direction parallel to the substrate surface.

  Although not shown in FIGS. 1 and 3, an etching gas or the like is introduced from the through-hole 8 (38), so that a desired amount of the first sacrificial layers 2 (32), 4 (34) and the second sacrificial layers are obtained. After removing the layer 6 (36), the surface of the sealing thin film 7 (37) may be covered with a thin film or the like, the through hole may be covered, and the first cavity 9 and the second cavity 10 may be sealed. .

  When the MEMS element is manufactured using a semiconductor process, the same structure can be obtained using various methods. Therefore, it should be noted that the MEMS device manufacturing method of the present invention is not limited to the flow shown in FIG.

(Second Embodiment)
FIG. 4 is a cross-sectional view showing a structural example of a MEMS element according to the second embodiment of the present invention. The MEMS element shown in FIG. 1 has a structure in which a plate-like electrode and a plate-like movable portion have a region where they overlap each other with a gap in a direction perpendicular to the substrate surface, and their main surfaces are parallel to each other. . On the other hand, the MEMS element shown in FIG. 4 includes a movable part 3 having a beam having a triangular cross-sectional structure (that is, a triangular prism shape), and the side surface of the electrode 5 faces the slope of the triangular movable part 3 in parallel. Have Also in this MEMS element, the movable part 3 and the electrode 5 have a region (an inclined surface of the movable part 3 and a side surface of the electrode 5 parallel to the same) with a gap in a direction perpendicular to the substrate surface. In the MEMS element shown in FIG. 4, the side of the movable part 3 is the lower side when viewed from the electrode 5 (when the electrode 5 is pushed downward, it comes into contact with the movable part 3). Therefore, the first cavity 9 and the second cavity 10 are spaces located respectively on the lower side and the upper side when viewed from the electrode 5. As in the MEMS element of FIG. 1, the inner surface a of the side wall A of the first cavity is located on the inner side in the direction parallel to the substrate surface than the inner surface b of the side wall B of the second cavity. . The effects obtained by this side wall structure are as described with reference to FIG.

  In the MEMS element shown in FIG. 4, the side wall A that defines the first cavity 9 is composed of one sacrificial layer. This is because a MEMS element is manufactured by a method of forming a layer to be an electrode on the surface of the BOX layer after forming the movable part 3 made of single crystal silicon using an SOI substrate. Since the other elements are as described above in connection with the first embodiment, the description thereof is omitted here.

  In the MEMS element shown in FIG. 4, the electrode 5 is symmetrical with respect to the left and right of the figure (the direction perpendicular to the direction in which the beam of the movable part 3 extends and the direction perpendicular to the thickness direction) with respect to the movable part 3. Is formed. Therefore, the reference position of the electrode 5 when determining A1 and B1 described with reference to FIGS. 12 and 13 in relation to the first embodiment is the upper end of the electrode 5 as shown in FIG. It will be a passing position.

  In the above, two types of MEMS elements having different shapes of the movable part have been described. The present invention can be similarly applied to a structure including a region in which an electrode and a movable part overlap each other through a minute gap in a direction perpendicular to the substrate surface.

(Third embodiment)
FIG. 5 is a cross-sectional view showing a structural example of a MEMS element according to the third embodiment of the present invention. In the illustrated MEMS element, a second sacrificial layer 12 is formed on a substrate 1, an electrode 5 is formed on the second sacrificial layer 12, a movable part 3 is provided above the electrode 5, One sacrificial layer 13, 16 is formed on the electrode 5. In the illustrated form, the sealing thin film 7 is provided on the first sacrificial layer 16.

  The electrode 5 and the movable part 3 have a region overlapping each other in a direction perpendicular to the surface of the substrate 1 through a minute gap (that is, in the illustrated form, the main surface of the electrode 5 and the main surface of the movable part 3 are Opposing each other so as to be parallel to the substrate surface), the vibrator is configured. The movable part 3 and the electrode 5 form a capacitance by a minute gap, and the movable part 3 is excited in a mode such as bending, spreading, or twisting by an electrostatic force generated by applying a voltage to the electrode 5.

In FIG. 5, the side of the movable part 3 when viewed from the electrode 5 is the upper side of the electrode 5. Therefore, in the illustrated form, the cavity located on the upper side when viewed from the electrode 5, that is, the space indicated by reference numeral 20 is the first cavity 20. Here, the first cavity 20 is formed by being surrounded by the electrode 5, the first sacrificial layers 13 and 16, and the sealing thin film 7. The first sacrificial layers 13 and 16 shown in FIG. 5 are portions that have not been removed by etching. As described in connection with the first embodiment, of the side walls defining the first cavity 20, the inner surface of the side wall A made of the first sacrificial layer 13 in contact with the electrode 5 (a in FIG. 5). Position) is determined in relation to the position of the inner surface b of the side wall B of the second cavity to be described later.

  In FIG. 5, a cavity located on the lower side when viewed from the electrode 5, that is, a space indicated by reference numeral 19 is a second cavity. Here, the second cavity 19 is formed by being surrounded by the substrate 1, the second sacrificial layer 12, and the electrode 5. The side wall B that defines the second sacrificial layer 19 is a portion of the second sacrificial layer 12 that has not been removed by etching. As described with reference to FIGS. 1, 12 and 13, how much the inner surface of the side wall A of the first cavity is positioned with respect to the inner surface of the side wall B of the second cavity. The mechanical force generated in the electrode 5 when mechanical pressure is applied to the sealing thin film 7 so that the gap between the electrode 5 and the movable portion 3 is maintained when mechanical pressure is applied to the stop film 7. It is preferably determined in consideration of stress.

  A through-hole 8 is formed in the sealing thin film 7. The through-hole 8 is formed at a desired position as an etching hole in order to form a cavity by removing the sacrificial layer. In the manufacture of the MEMS element having the form shown in FIG. 5, the first sacrificial layers 13 and 16 and the second sacrificial layer 12 are formed of materials having different etching rates when the sacrificial layer is removed. More specifically, the material of each sacrificial layer is selected so that the etching rate of the second sacrificial layer 12 is higher than that of the first sacrificial layers 13 and 16. As a result, the etching gas or etchant introduced from the through-hole 8 removes the first sacrificial layer 16 and further passes through the opening 12 to sequentially pass the first sacrificial layer 13 and the second sacrificial layer 12. If removed, the second cavity 19 has a larger dimension in the direction parallel to the substrate surface than the first cavity 20. That is, the inner surface a of the side wall A that defines the first cavity 20 is located on the inner side in a direction parallel to the substrate surface than the inner surface b of the side wall B that defines the second cavity 19.

  When the MEMS element having such a structure is sealed with a resin transfer mold as shown in FIG. 2A, the pressure applied to the sealing thin film 7 passes through the side wall A, and the first sacrifice on the lower side. The electrode 5 is pushed in at the contact point between the layer 13 and the electrode 5. Since the space below the electrode 5 is a cavity at the contact point between the side wall A and the electrode 5, the electrode 5 is pushed down as it is. Therefore, a stress component acting on the side opposite to the movable portion 3 is generated in the electrode 5, and as a result, collision between the electrode 5 and the movable portion 3 is avoided. That is, in the configuration of FIG. 5, when a molding pressure is applied to the sealing thin film 7, the sealing thin film 7 and the electrode 5 are integrally displaced in the thickness direction and behave as indicated by the broken line in FIG. Therefore, the electrode 5 is displaced in a direction away from the movable portion 3 by setting the position of the side wall A of the first cavity to the inside of the position of the side wall B of the second cavity.

  FIG. 6 is a sectional view showing another structural example of the MEMS element according to the third embodiment of the present invention. The MEMS element shown in FIG. 6 differs from the MEMS element shown in FIG. 5 in that a through-hole 18 is formed in the substrate 1 as an etching hole for removing a sacrificial layer and forming a cavity. In this embodiment, the first sacrificial layers 13 and 16 and the second sacrificial layer 12 are formed of the same or the same kind of material. The MEMS element of this form is manufactured by introducing an etching gas or an etchant from the through-hole 18 and removing the second sacrificial layer 12, the first sacrificial layer 13, and the first sacrificial layer 16 in this order. Can do.

  By introducing an etching gas or the like from the through-hole 18, the exposure time to the etching gas or the like becomes longer in the second sacrificial layer 12. Therefore, even when all the sacrificial layers are made of the same material, the second cavity 19 has a larger dimension in the direction parallel to the substrate surface than the first cavity 20. That is, the inner surface a of the side wall A that defines the first cavity 20 is located more inward in the direction parallel to the substrate surface than the inner surface b of the side wall B that defines the second cavity 19. Thereby, when the surface of the sealing thin film 7 and the side surface of the element are mold-sealed with resin, the collision between the electrode 5 and the movable portion 3 is avoided according to the mechanism described with reference to FIG.

  Although not shown in FIG. 5, an etching gas or the like is introduced from the through-hole 8 to remove a desired amount of the first sacrificial layers 13 and 16 and the second sacrificial layer 12. The first cavity 20 and the second cavity 19 may be sealed by covering the surface with a thin film or the like and covering the through-hole. Similarly, although not shown in FIG. 6, an etching gas or the like is introduced from the through hole 18 to remove a desired amount of the first sacrificial layers 13 and 16 and the second sacrificial layer 12, and then the substrate 1. The first cavity 20 and the second cavity 19 may be sealed by covering the surface (the exposed surface located on the lower side in the figure) with a thin film and covering the through-hole.

  5 and 6, the inner surface of the first sacrificial layer 16 is located on the inner side in the direction parallel to the substrate surface than the inner surface of the first sacrificial layer 13. When the first sacrificial layer is composed of two or more layers, the positional relationship between the inner surfaces of these layers is not particularly limited. For example, in the MEMS element shown in FIG. 5 and FIG. 6, the inner surface a of the side wall A composed of the first sacrificial layer 13 in contact with the electrode 5 is more than the inner surface b of the side wall B composed of the second sacrificial layer 12. As long as it is located inside in the direction parallel to the substrate surface, the inside surface of the upper first sacrificial layer 16 is located outside in the direction parallel to the substrate surface rather than that of the lower first sacrificial layer 13. It's okay.

  5 and 6 show structures in which the main surfaces of the electrode and the movable part are perpendicular to the substrate surface and are parallel to each other. 5 and FIG. 6 may have another structure in which the electrode and the movable portion have regions that overlap each other with a gap in a direction perpendicular to the substrate surface.

  The MEMS element according to the present invention includes an electrode having a region that overlaps with a gap in a direction perpendicular to the substrate surface and a movable part. After sealing with a sealing thin film, mechanical pressure is applied during resin molding. Even if added, high reliability is achieved in that the collision between the electrode and the movable part is avoided. Therefore, the MEMS element of the present invention can be widely applied to devices such as a switch element, a resonator, a filter, an oscillator, a gyroscope, a pressure sensor, and a mass detection element, and electronic equipment using them.

1 Substrate 2, 4, 6, 12, 13, 16 Sacrificial layer 3 Movable part (beam)
5 Electrode 7 Sealing thin film 8, 18 Through hole (etching hole)
9, 10, 19, 20 Cavity 11 Resin A Side wall B defining first cavity Side wall a defining second cavity a Inner surface b of side wall A b Inner surface of side wall B

Claims (11)

Having a substrate and a sealing thin film,
A movable part of a beam structure that performs mechanical vibration and an electrode positioned in proximity to the movable part are provided between the substrate and the sealing thin film, and the movable part and the electrode are perpendicular to the substrate surface. Having regions overlapping each other with a gap in the direction,
A first cavity and a second cavity separated by the electrode are formed between the substrate and the sealing thin film,
The first cavity is located on the movable portion side in a direction perpendicular to the substrate surface when viewed from the electrode in the region where the movable portion and the electrode overlap.
The second cavity is located on a side opposite to the movable part in a direction perpendicular to the substrate surface when viewed from the electrode in a region where the movable part and the electrode overlap.
When a force in a direction perpendicular to the substrate surface is applied to the electrode, a position that is most displaced in a direction perpendicular to the substrate surface in a region facing the movable part of the electrode is defined as a reference position O.
A1 is a distance from the reference position O to the inner surface of the side wall A in contact with the electrode of the first cavity in a direction parallel to the substrate surface and perpendicular to the beam constituting the movable part,
The distance from the reference position O to the inner surface of the side wall B in contact with the electrode of the second cavity in the direction parallel to the substrate surface and perpendicular to the beam constituting the movable part is defined as B1. sometimes,
3.2 ≧ B1 / A1 ≧ 1.5 ,
MEMS element.   2. The MEMS device according to claim 1, wherein an inner surface of the entire side wall of the first cavity is located on an inner side in a direction parallel to the substrate surface than an inner surface of the entire side wall of the second cavity.   2. The first cavity according to claim 1, wherein the first cavity is formed by removing a first sacrificial layer, and the second cavity is formed by removing a second sacrificial layer. MEMS element. In the same etching process, the second sacrificial layer is first removed and then the first sacrifice layer is removed, the second cavity and the first cavity is formed, according to claim 3 MEMS element. The first sacrificial layer and the second sacrificial layer are made of different materials, and the etching rate of the second sacrificial layer is higher than the etching rate of the first sacrificial layer. A material is selected, and in the same etching process, the first sacrificial layer is removed first, then the second sacrificial layer is removed to form the first cavity and the second cavity. The MEMS element according to claim 3 . Outside of the encapsulation thin film is molded with resin, MEMS device according to any one of claims 1-5. Having a substrate and a sealing thin film,
A movable part that performs mechanical vibration and an electrode positioned in proximity to the movable part are provided between the substrate and the sealing thin film, and the movable part and the electrode have a gap in a direction perpendicular to the substrate surface. Having regions that overlap each other,
A first cavity and a second cavity separated by the electrode are formed between the substrate and the sealing thin film,
The first cavity is located on the movable portion side in a direction perpendicular to the substrate surface when viewed from the electrode in the region where the movable portion and the electrode overlap.
The second cavity is located on a side opposite to the movable part in a direction perpendicular to the substrate surface when viewed from the electrode in a region where the movable part and the electrode overlap.
When a force in a direction perpendicular to the substrate surface is applied to the electrode, a position that is most displaced in a direction perpendicular to the substrate surface in a region facing the movable part of the electrode is defined as a reference position O.
A1 is a distance from the reference position O to the inner surface of the side wall A in contact with the electrode of the first cavity in a direction parallel to the substrate surface and perpendicular to the beam constituting the movable part,
The distance from the reference position O to the inner surface of the side wall B in contact with the electrode of the second cavity in the direction parallel to the substrate surface and perpendicular to the beam constituting the movable part is defined as B1. sometimes,
3.2 ≧ B1 / A1 ≧ 1.5 , a method for manufacturing a MEMS device,
A method for manufacturing a MEMS device, comprising: forming the first cavity by removing a first sacrificial layer; and forming the second cavity by removing a second sacrificial layer. Forming the first cavity and forming the second cavity are performed by the same etching process, in which the second sacrificial layer is first removed, and then the first sacrificial layer is formed. The method for manufacturing a MEMS device according to claim 7 , wherein the layer is removed. Selecting the materials of the first sacrificial layer and the second sacrificial layer such that the etching rate of the second sacrificial layer is greater than the etching rate of the first sacrificial layer;
Forming the first cavity and forming the second cavity are performed by the same etching process, wherein in the etching process, the first sacrificial layer is first removed, and then the second sacrificial layer is formed. The method for manufacturing a MEMS device according to claim 7 , wherein the layer is removed. Having a MEMS device according to any one of claims. 1 to 6, MEMS oscillator. The electronic device which has at least any one of the MEMS element of any one of Claims 1-6 , and the MEMS oscillator of Claim 10 .

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